Liquid crystal display device with controlled viewing angle panel and driving method thereof

ABSTRACT

A liquid crystal display device and a method for driving the same are provided. The liquid crystal display device includes a first liquid crystal layer selectively driven by a first electric field in a first direction; and a second liquid crystal layer selectively driven a second electric field in a second direction, the second direction being different from the first direction.

This application is a divisional of parent application Ser. No.11/453,935 filed Jun. 16, 2006 now U.S. Pat. No. 7,948,582 whichapplication claims priority under 35 U.S.C. §119(a) on PatentApplication No. 10-2005-0096593 filed in Korea on Oct. 13, 2005, theentire contents of each application being hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device anddriving method thereof. Particularly the present invention relates to aliquid crystal display device in which a Wide Viewing Angle mode and aNarrow Viewing Angle mode can be easily selected by a user, and adriving method thereof.

2. Description of the Related Art

In general, a liquid crystal display device displays image bycontrolling optical transmittance of liquid crystal materials. This isdone by injecting liquid crystal materials between two substrates (onehas common electrode and the other has pixel electrode) and applying anelectric field to the liquid crystal through electrodes facing eachother with the liquid crystal therebetween.

A liquid crystal display device according to the direction of theelectric field applied to the liquid crystal layer, can be categorizedinto a vertical electric field type and a horizontal electric fieldtype.

The vertical electric field type LCD device drives the liquid crystallayer between a pixel electrode and a common electrode through avertical electric field (vertical to an LCD panel surface). The commonelectrode of the upper substrate and the pixel electrode of the lowersubstrate are all transparent electrodes. Thus, high aperture ratio canbe easily provided However, a disadvantage of this type LCD device isthat a viewing angle range is narrowed to about 90°. This results fromthe movement of the liquid crystal which affects the light passingthrough the substrate in an oblique direction as the liquid crystalmoves in the vertical direction to the substrate due to verticalelectric field.

The horizontal electric field type LCD device is so-calledIn-Plane-Switching mode (IPS) LCD device. The IPS LCD device is drivenby a horizontal electric field (horizontal direction to an LCD panelsurface) between the pixel electrode and the common electrode arrayed onthe lower substrate. In this mode, there is hardly a movement in thevertical direction as the liquid crystal is driven in the horizontaldirection mainly. Therefore, the device has an advantage of having awide viewing angle range of about 160°. Therefore, there is littleeffect on the light which passes through the substrate in an obliquedirection.

FIGS. 1A and 1B illustrate a simplified related art IPS mode liquidcrystal display device. In particular, it illustrates that the commonelectrode and the pixel electrode are arrayed in the pixel area. FIGS.1A and 1B show that the IPS mode liquid crystal display device has thecommon electrode, the pixel electrode, upper and lower part substratesand polarizers.

According to FIGS. 1A and 1B, the IPS mode liquid crystal display devicecomprises a thin film transistor substrate (lower substrate) and a colorfilter substrate (upper substrate) which faces each other with a liquidcrystal layer 10 therebetween and a spacer which maintains a cell gapbetween the two substrates.

The thin film transistor substrate comprises a gate line and a data linedefining a pixel unit on a lower substrate 1, a thin film transistorformed at the crossing point of the gate line and the data line, acommon electrode 5 and a pixel electrode 7 forming a horizontal electricfield, and an alignment layer deposited on the common electrode and apixel electrode for the initial alignment of the liquid crystal.

The color filter substrate comprises a color filter to present colors onthe upper substrate 11, a black matrix to prevent light leakage betweenthe neighboring color filters and the alignment layer deposited on thecolor filter and the black matrix for the initial alignment of theliquid crystal.

A lower polarizer 3 and an upper polarizer 13 polarizing incident lightfrom a back light unit are adhered on the outside of the upper and lowersubstrates 1, 11. As shown in FIG. 1, the transmitting axes (polarizingaxis of the polarizer) of the lower polarizer 3 and the upper polarizer13 are perpendicular to each other. The linearly polarized lightpolarized by the lower polarizer 3 is transmitted into the liquidcrystal materials. If the power is off, then the liquid crystalmaintains its initial state. Therefore, the phase change due to theliquid crystal does not occur, and the polarizing direction does notchange and transmits is the light. The direction of the linearlypolarized light is perpendicular to the polarization axis of the upperpolarizer 13. Therefore the linearly polarized light cannot pass throughthe upper polarizer 13. In other words, it shows the NB condition(Normally Black: dark screen appears when the power is off).

When an electric field is applied between the upper and lower substrates1, 11, a liquid crystal 10 changes its alignment state according to thesupplied signal. The operation of dark and bright display modes of theliquid crystal at the IPS mode would be described in detail hereinafter.

The display of dark screen is described with reference to FIG. 1A. Thelight polarized through the lower polarization substrate 3 enters intoliquid crystal molecules 10A. The liquid crystal molecules are arrayedin parallel in an initial alignment direction as the electric field isnot formed. A long axis of the liquid crystal molecules 10A is parallelto the transmitting axis of the polarizer 3. In addition, the long axisis initially aligned by the alignment layer to be in 90° as shown in theFIG. 1A. As a result, the polarization status does not change as thephase delay does not happen even if the polarized light enters into theliquid crystal molecules 10A. The light which enters into the liquidcrystal molecules 10A is blocked because it cannot pass through theupper polarizer 13 having a transmitting axis perpendicular to thepolarization direction of the lower polarizer 3. Therefore the liquidcrystal display device shows a dark screen.

The display of a bright screen is described with reference to FIG. 1B.An electric field is formed between the electrodes 5, 7 and the liquidcrystal molecules are rotated by the electric field. As a result, theliquid molecule is twisted. In general the twisted liquid molecules 10Bare twisted to be in an angle of 45° to the transmitting axis of thelower polarizer 3 on average. The light polarized through the lowerpolarizer 3 has a phase delay as it passes through the twisted liquidcrystal molecules 10B. The phase of the light polarized by the lowerpolarizer 3 is delayed by λ/2 along with the twisted liquid crystal 10B.Therefore, the light axis of the light incident from the lower polarizer3 changes to 90°. The light axis of the light which has passed throughthe twisted liquid crystal molecules 10B is parallel to the transmittingaxis of the upper polarizer 13 and therefore passes through the upperpolarizer 13. Accordingly, the liquid crystal display device shows abright screen.

The IPS mode liquid crystal display device has a wide viewing anglecompared to the vertical electric field mode liquid crystal displaydevice. The device having a wide viewing angle has an advantage in thatthe viewer can see images within a wide viewing angle range. However, insome cases such as using computers for personal purpose or conducting asecurity-required work at banks or insurance companies, a narrow viewingangle LCD device is preferred.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a liquid crystaldisplay device which can switch from the wide viewing angle mode to thenarrow viewing angle mode easily according to the work environment ofthe users.

In order to achieve the above mentioned purpose, a liquid crystaldisplay device, as embodied, a first electrode group to selectivelyapply a horizontal electric field; a first panel including a firstliquid crystal layer driven by the horizontal electric field; a secondelectrode group below or above the first panel to selectively apply avertical electric field, and a second panel including a second liquidcrystal layer driven by the vertical electric field.

In another aspect of the present invention, as embodied, a method fordriving a liquid crystal display device, the method comprising:displaying a video image on a first panel by selectively applying ahorizontal electric field to a first liquid crystal layer of the firstpanel of the liquid crystal display device; and selectively applying avertical electric field to a second liquid crystal layer of the secondpanel of the liquid crystal display device.

In another aspect of the present invention, as embodied, a liquidcrystal display device comprising: a first liquid crystal layerselectively driven by a first electric field in a first direction; and asecond liquid crystal layer selectively driven a second electric fieldin a second direction, the second direction being different from thefirst direction.

In another aspect of the present invention, as embodied, a method fordriving a liquid crystal display device, the method comprising:selectively applying a first electric field in a first direction on afirst liquid crystal layer of the liquid crystal display device;selectively applying a second electric field in a second direction on asecond liquid crystal layer of the liquid crystal display device, thesecond direction being different from the first direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate the operation of the related art for In PlaneSwitching mode liquid crystal display panel.

FIG. 2 illustrates the structure of a liquid crystal display deviceaccording to an embodiment of the present invention.

FIGS. 3A and 3B are perspective view and cross sectional viewillustrating operation of wide viewing angle mode of the liquid crystaldisplay device according to an embodiment of the present invention whendisplaying a dark image.

FIGS. 4A and 4B are perspective view and cross sectional viewillustrating operation of narrow viewing angle mode of the liquidcrystal display device according to an embodiment of the presentinvention when displaying a dark image.

FIG. 5 is a graph indicating a transmission factor with respect to theviewing angle when displaying a dark image on the liquid crystal displaydevice.

FIGS. 6A and 6B are perspective view and cross sectional viewillustrating the operation of a wide viewing angle mode of the liquidcrystal display device according to an embodiment of the presentinvention when displaying a bright image.

FIGS. 7A and 7B are perspective view and cross sectional viewillustrating operation of narrow viewing angle mode of the liquidcrystal display device according to an embodiment of the presentinvention when displaying a bright image.

FIG. 8 is a graph indicating a transmission factor with respect to theviewing angle when displaying a bright image on the liquid crystaldisplay device.

FIG. 9 is a graph indicating a contrast ratio with respect to theviewing angle at the wide and narrow viewing angle modes of the liquidcrystal display device according to an embodiment of the presentinvention.

FIGS. 10A and 10B are graphs indicating a contrast ratio with respect tothe left/right, top/bottom viewing angles at the wide and narrow viewingangle modes of the liquid crystal display device according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The purposes and advantages of the present invention will be describedthrough the preferred embodiment of the present invention with referenceto the attached drawings. The preferred embodiment of the presentinvention will be described referring to FIG. 2 through 10A.

FIG. 2 illustrates a simplified structure of the liquid crystal displaydevice according to an embodiment of the present invention. Inparticular, FIG. 2 shows a common electrode and pixel electrode arrayedin parallel on one pixel area.

Referring to FIG. 2, the illustrated liquid crystal device comprises afirst panel part (A) which operates in an In Plane Switching mode toprovide a wide viewing angle mode by driving the liquid crystal with ahorizontal electric field, and a second panel part (B) which operates inan Electrically Controlled Birefringence (ECB) mode to change theviewing angle range by driving the liquid crystal with a verticalelectric field.

The first panel part (A) which is driven in the IPS mode includes afirst substrate 21 and a second substrate 31, a spacer to maintain cellgap between the first and second substrates 21, 31 and a first liquidcrystal layer 50 filling the cell gap.

A gate line and a data line defining a pixel unit, a thin filmtransistor which is formed on the crossing area of the gate and dataline, a common electrode 25 and a pixel electrode 27 which are parallel,and a first alignment layer 28 coated on the common and pixel electrodefor initial alignment of the first liquid crystal layer 50 are formed onthe upper part of the first substrate 21. A lower polarizer 23 is formedat the lower part of the first substrate 21. A second alignment layer 36is formed at the lower part of the second substrate 31 for initialalignment of the first liquid crystal layer 50.

The first and second alignment layers 28, 36 define the initialalignment direction of the first liquid crystal layer 50. The firstliquid crystal layer 50 is aligned such that the long axis direction ofthe first liquid crystal is parallel to the first and second alignmentlayers 28, 36.

The common electrode 25 and the pixel electrode 27 which are arrayed inparallel to form a horizontal electric field when a voltage is suppliedto drive the first liquid crystal layer 50. The first liquid crystallayer 50 driven by the horizontal electric field provides a wide viewingangle range.

The lower polarizer 23 selectively transmits the component of theincident light from the back light unit, which is parallel to thepolarization direction of the lower polarizer 23.

The second panel part (B) driven by ECB mode includes a second substrate31 and a third substrate 41, a spacer maintaining the cell gap betweenthe two substrates and a second liquid crystal layer 60 filling the cellgap.

A first electrode 35 which corresponds to the lower electrode and athird alignment layer 38 coated on the first electrode to align thesecond liquid crystal layer 60 are formed on the upper part of thesecond substrate 31. A second electrode 45 which corresponds to theupper part electrode and a forth alignment layer 48 coated under thesecond electrode to align the second liquid crystal layer 60 are formedon the lower part of the third substrate 41. An upper part polarizer 43is formed on the upper part of the third substrate 41.

A color filter for presenting the colors and a black matrix forpreventing light leakage can be formed between the lower part of thethird substrate 41 and the second electrode 45.

The third and the forth alignment layers 38, 48 set up the initialalignment direction of the second liquid crystal layer 60. The alignmentdirection of the third and forth alignment layers 38, 48 is identical tothe alignment direction of the first and second alignment layers whereinthe second liquid crystal layer 60 is aligned to have an identicaldirection of the alignment direction of the first liquid crystal layer50.

The first electrode 35 and the second electrode 45 formed at theupper/lower part of the second panel part (B) generate a verticalelectric field when a power is applied and drive the second liquidcrystal layer 60. The liquid crystal layer 60 driven at the verticalelectric field can switch between the narrow viewing angle mode and thewide viewing angle mode.

The upper polarizer 43 selectively transmits the light from the firstpanel part (A) having the direction parallel to the transmitting axis(Y) of the upper polarizer 43. The transmitting axis (Y) of the upperpolarizer 43 is perpendicular to the transmitting axis (X) of the lowerpolarizer 23. The liquid crystal display device according to anembodiment of the present invention may have a middle polarizer on theupper or lower part of the second substrate 31. In case of having themiddle polarizer, the transmitting axis of the middle polarizer and thetransmitting axis of the lower polarizer 23 are perpendicular to eachother. The transmitting axis (X) of the lower polarizer 23 and thetransmitting axis of the upper polarizer 43 are parallel. The detaileddescription will be given referring to FIGS. 3-7.

As illustrated in FIG. 2, the liquid crystal display device according toan embodiment of the present invention is driven in the wide viewingangle mode and the narrow viewing angle mode using the first panel part(A) and the second panel part (B). The first panel part (A) has thecharacteristics of the wide viewing angle and the second panel part (B)is selectively driven by a vertical electric field to provide the wideviewing angle mode and the narrow viewing angle mode according to theon-off of the vertical electric field.

The detailed description of the wide viewing angle mode and the narrowviewing angle mode will be given referring to FIGS. 3-8.

FIGS. 3A and 3B are a perspective view and a cross sectional view of theliquid crystal display device according to an embodiment of the presentinvention to illustrate the operation of the wide viewing angle modewhen the screen shows dark image.

Referring to the FIGS. 3A and 3B, the first panel part (A) driven in IPSmode maintains the initial alignment direction so that the long axisdirection of the first liquid crystal layer 50 is substantiallyperpendicular (or parallel) to the transmitting axis (X) of the lowerpolarizer 23. The LCD can operate in an NB (normally Black) condition byarranging the transmitting axis (X) of the lower polarizer 23perpendicular to the transmitting axis (Y) of the upper polarizer 43.

The incident light from the back light unit is linearly polarized to beparallel to the transmitting axis (x) of the lower polarizer 23, if thehorizontal electric field is not applied to the first panel part A. Thispolarized light passes through the first liquid crystal layer 50.However, the long axis direction of the first liquid crystal layer 50 is90° (or parallel) and therefore there is no phase delay. The polarizedlight maintains the polarization condition of the lower polarizer 23.Subsequently, the light passes through the transparent second substrate31. The light which passes through the second substrate 31 then passesthe second liquid crystal layer 60. As there is no electric field at thesecond panel part (B), the second liquid crystal layer 60 maintains theinitially alignment direction. Therefore, the light which passes thesecond liquid crystal layer 60 does not have the phase delay andmaintains the polarization condition of the lower polarizer 23. Thelight maintaining the polarization condition of the lower polarizer 23is blocked by the upper polarizer 43 it is perpendicular to thetransmitting axis (Y) of the upper polarizer 43. As a result, the liquidcrystal display device shows a dark image. As illustrated in FIG. 3B,the light is blocked regardless of the viewing directions (frontdirection: R, oblique direction: S1, S2). Therefore a black color isshown evenly throughout the wide viewing angle range.

As another embodiment of the present invention, a middle polarizer canbe further included in the upper or lower part of the second substrate31. In case of having the middle polarizer, the transmitting axis of thelower polarizer 23 and the transmitting axis of the middle polarizer areperpendicular to each other and the transmitting axis of the lowerpolarizer 23 and the transmitting axis of the upper polarizer 43 areparallel to each other. Furthermore, only the component of the lightincident from the back light unit parallel to the transmitting axis ofthe lower polarizer 23 can pass through the lower polarizer 23. Thelight which transmitted the lower polarizer 23 also passes through thefirst liquid crystal layer 50 which has no phase delay. Therefore, thelight maintains the polarization condition of the lower polarizer 23.The light which passes through the first liquid crystal layer 50 andthus maintains the polarization condition of the lower polarizer 23cannot pass through the middle polarizer which is formed vertical to thetransmitting axis of the lower polarizer 23. Finally, a dark image isdisplayed out of the upper polarizer 43. The final display result of theliquid display device is identical to FIGS. 3A and 3B even if the middlepolarizer is added.

FIGS. 4A and 4B are a perspective view and a cross sectional view of theliquid crystal display device according to an embodiment of the presentinvention to illustrate the operation of the narrow viewing angle modewhen the screen shows a dark image. Referring to the FIGS. 4A and 4B,the first panel part (A) driven in IPS mode maintains the initialalignment direction so that the long axis direction of the first liquidcrystal layer 50 is substantially perpendicular (or parallel) to thetransmitting axis (X) of the lower polarizer 23.

The LCD can operate in an NB (normally Black) condition by arranging thetransmitting axis (X) perpendicular to the transmitting axis of theupper polarizer (Y). The incident light from the back light unit islinearly polarized to be parallel to the transmitting axis (x) of thelower polarizer 23 if the horizontal electric field is not applied tothe first panel part (A). This polarized light passes through the firstliquid crystal layer 50. At that time, the long axis direction of thefirst liquid crystal layer 50 is 90° (or parallel) and therefore thereis no phase delay. The polarized light maintains the polarizationcondition of the lower polarizer 23. After that, the light passesthrough the transparent second substrate 31. The light which passesthrough the second substrate 31 also passes through the second panelpart (B) in which the vertical field is applied.

In particular, the liquid crystal molecules of the second liquid crystallayer 60 is rotated to have a certain oblique angle against the plane ofthe second substrate 31 as the vertical electric field is applied to thesecond liquid crystal layer 60. Accordingly, the light which passesthrough the first liquid crystal layer 50 and the second substrate 31and maintains the polarization condition of the lower polarizer 23passes through the second liquid crystal layer 60. The condition of thelight which passes through the second liquid crystal layer 60 will bedescribed referring to the FIG. 4B.

Referring to the FIG. 4B, the light which passes through the secondliquid crystal layer 60 has a phase difference according to thetransmitting direction of the light. The light transmitted in the frontdirection (R) does not have the phase delay even if the light istransmitted in the twisted second liquid crystal layer 60 as shown inFIGS. 3A and 3B. This is because the twisted condition of the liquidcrystal layer does not affect the light in the front direction (R). As aresult, the light maintains the polarization condition of the lowerpolarizer 23. The light transmitted in the front direction (R) does nothave the same polarizing direction with the transmitting axis (Y) of theupper polarizer 43. Therefore, it is blocked by the upper polarizer 43and the liquid crystal display device displays a dark image. However,when the light is transmitted in the oblique directions (S1, S2), thephase delay occurs due to the twisted condition of the second liquidcrystal layer 60. As a result, the polarized condition of the lightchanges due to the phase delay to have the component parallel to thetransmitting axis (Y) of the upper polarizer 43 (e.g., the lighttransmitted in oblique directions S1 and S2), which causes lightleakage. If the electric field is not formed at the first panel part (A)but is formed at the second panel part (B), the light leaks at theoblique directions (S1, S2). Since the light leakage may be undesirable,when display a dark image, the vertical electric field can be disabledso that it will not be applied to the second panel part (B) to preventthe light leakage.

A middle polarizer can also be further included at the upper or lowerpart of the second substrate 31 as mentioned in the previous embodiment.In case of having the middle polarizer, the transmitting axis of thelower polarizer 23 and the transmitting axis of the middle polarizer areperpendicular to each other, while the transmitting axis of the lowerpolarizer 23 and the transmitting axis of the upper polarizer 43 areparallel. Furthermore, only the component of the light incident from theback light unit which is parallel to the transmitting axis of the lowerpolarizer 23 will pass through the lower polarizer 23. The light whichpasses through the lower polarizer 23 also passes the first liquidcrystal layer 50 without phase delay. Therefore, the light maintains thepolarization condition of the lower polarizer 23. The light which passesthe first liquid crystal layer 50 and thus maintains the polarizationcondition of the lower polarizer 23 cannot pass through the middlepolarizer which is perpendicular to the transmitting axis of the lowerpolarizer 23. Finally, a dark image is displayed out of the upperpolarizer 43. The final display result of the liquid display device isidentical to FIGS. 4A and 4B even if the middle polarizer is added.

FIG. 5 shows the transmitting ratio of the liquid crystal display devicewith respect to the viewing angle when displaying a dark imageillustrated in FIG. 3A to 4B. As shown in FIG. 5, the second panel part(B) which is ECB mode has a transmitting ratio illustrated as theα-curve when the vertical electric field is off (wide viewing anglemode) and has a transmitting ratio illustrated as the β-curve when thevertical electric field is on (narrow viewing angle mode).

The α-curve shows that a normal dark image can be displayed on a wideviewing angle range as the light is blocked regardless of thetransmitting direction when the vertical electric field applied to thesecond panel part (B) is off (wide viewing angle mode). The β-curveshows that the light passes through the upper polarizer and leaks. Thereason for the leakage is because the light which passes in the obliquedirections changes in its polarization characteristic due to the twistedsecond liquid crystal layer 60, when the vertical electric field appliedto the second panel part (B) is on (narrow viewing angle mode). Thenormal dark image can only be displayed between the viewing angle of+20°˜−20° (viewing angle below 40°) as the transmitting ratio is below0.05%.

FIGS. 6A and 6B are a perspective view and a cross sectional view of theliquid crystal display device according to an embodiment of the presentinvention to illustrate the operation of the wide viewing angle modewhen the screen shows a bright image. Referring to FIGS. 6A and 6B, thefirst panel part (A) which is driven in IPS mode applies a horizontalelectric field to the first liquid crystal layer 50 when displaying abright image. When the horizontal electric field is applied to the firstpanel part (A), the first liquid crystal layer 50 rotates due to thehorizontal electric field between a common electrode and a pixelelectrode. Therefore, the first liquid crystal layer 50 is twisted. Thetwist angle of the liquid crystal cell 50 is 45° on average to thepolarization direction of the lower polarizer 23.

The component of the light incident from the back light unit parallel tothe transmitting axis (X) of the lower polarizer 23 passes through thelower polarizer 23 and the first liquid crystal layer 50 when thehorizontal electric field is applied to the first panel part (A). Thelight passes the first liquid crystal layer 50 which is 45° on averageto the transmitting axis (X) of the lower polarizer 23 and the phase ofpolarization of the light is delayed by λ/2. Thus the polarization ofthe light (polarization to X axis direction) is changed to 90°. Thepolarized light then passes through the transparent second substrate 31.The light which passes the second substrate 31 maintains the changedpolarization direction as there is no electric field applied at thesecond panel part (B). In other words, the light passing through thesecond liquid crystal layer 60 does not have the phase delay andmaintains the polarization condition which has changed in 90° from thepolarization condition after passing through the lower polarizer 23. Thelight maintaining the above mentioned condition passes through the upperpolarizer 43 as the light polarized direction and the transmitting axis(Y) of the upper polarizer 43 are parallel. Therefore, the liquidcrystal display device displays a bright image. As shown in FIG. 6B, thelight passes regardless of the transmitting direction (front direction:R, oblique directions: S1, S2) so that a bright image is displayed inthe wide viewing angle mode.

A middle polarizer can also be further included at the upper or lowerpart of the second substrate 31 as mentioned in the previousembodiments. In case of having the middle polarizer, the transmittingaxis of the lower polarizer 23 and the transmitting axis of the secondpolarizer are perpendicular to each other while the transmitting axis ofthe lower polarizer 23 and the transmitting axis of the upper polarizer43 are parallel. Furthermore, only the component of the light incidentfrom the back light unit which is parallel to the transmitting axis ofthe lower polarizer 23 will pass through the lower polarizer 23. Thelight passes through the first liquid crystal layer 50 which is 45° onaverage to the transmitting axis (X) of the lower polarizer 23 and itsphase is delayed by λ/2. Therefore, the polarization of the light(polarization to X axis direction) is changed to 90°. The light changedto 90° at the initial incident polarization direction passes through thetransparent second substrate 31. The light which passes through thesecond substrate 31 maintains the changed polarization direction asthere is no electric field applied at the second panel part (B). Inother words, the light passing through the second liquid crystal layer60 does not have the phase delay and maintains the polarizationcondition which has changed in 90° from the polarization condition afterpassing the lower polarizer 23. The light maintaining the abovementioned condition passes through the upper polarizer 43 as the lightaxis and the transmitting axis (Y) of the upper polarizer 43 areparallel. Therefore, the liquid crystal display device displays a brightimage. The final display result of the liquid display device isidentical to FIGS. 6A and 6B even if the middle polarizer is added.

FIGS. 7A and 7B are a perspective view and a cross sectional view of theliquid crystal display device according to an embodiment of the presentinvention to illustrate the operation of the narrow viewing angle modewhen the screen shows a bright image. Referring to FIGS. 7A and 7B, thefirst panel part (A) which is driven in IPS mode applies the horizontalelectric field to the first liquid crystal layer 50 when displaying abright image. When the horizontal electric field is applied to the firstpanel part (A), the first liquid crystal layer 50 rotates due to thehorizontal electric field between a common electrode and a pixelelectrode. Therefore, the first liquid crystal layer 50 is twisted. Thetwist angle of the liquid crystal cell 50 is 45° on average to thepolarization direction of the lower polarizer 23.

The component of the light incident from the back light unit parallel tothe transmitting axis (X) of the lower polarizer 23 passes through thelower polarizer 23 and the first liquid crystal layer 50 when thehorizontal electric field is applied to the first panel part (A). Thelight transmits the first liquid crystal layer 50 which is 45° onaverage to the transmitting axis (X) of the lower polarizer 23 and itsphase is delayed by λ/2. Therefore, the polarization of the light(polarization to X axis direction) is changed to 90°. The light changedto 90° from the initial incident polarization direction passes throughthe transparent second substrate 31. The light which passes through thesecond substrate 31 then passes through the second panel part (B)applied by a vertical electric field so it can operates in the narrowviewing angle mode.

In particular. the second liquid crystal layer 60 is rotated to have acertain oblique angle against the second substrate 31 as the verticalelectric field is applied to the second liquid crystal layer 60. Thelight which passes through the first liquid crystal layer 50 and thesecond substrate 31 and which maintains the polarization condition ofthe lower polarizer 23 passes the second liquid crystal layer 60. Thecondition of the light which passes the second liquid crystal layer 60will be described referring to the FIG. 7B.

Referring to the FIG. 7B, the light which passes the second liquidcrystal layer 60 has a phase difference according to the transmittingdirection of the light. The light transmitted in the front direction (R)does not have the phase delay even if the light passes the twistedsecond liquid crystal layer 60. This is because the twisted condition ofthe liquid crystal layer does not affect the light transmitted in thefront direction (R). As a result, the light maintains the polarizationcondition by the lower polarizer 23. The light transmitted in the frontdirection (R) has the same direction with the transmitting axis (Y) ofthe upper polarizer 43 so it passes through the upper polarizer 43.Therefore, the liquid crystal display device displays a bright image.However, when the light is transmitted in oblique directions (S1, S2),the phase delay occurs due to the twisted condition of the second liquidcrystal layer 60 by the vertical electric field. As a result, thepolarized condition changes due to the phase delay. Therefore, thecomponent of the light perpendicular to the transmitting axis (Y) of theupper polarizer 43 (e.g., the light transmitted in oblique directions S1and S2) will be blocked by the upper polarizer 43. If the horizontalelectric field is formed at the first panel part (A) and the verticalelectric field formed at the second panel part (B), the light is blockedat the oblique directions (S1, S2). Thus the viewing angle showing thenormal image is narrowed.

A middle polarizer can also be further included at the upper or lowerpart of the second substrate 31 as mentioned in the previous preferredembodiments. In case of having the middle polarizer, the transmittingaxis of the lower polarizer 23 and the transmitting axis of the middlepolarizer are perpendicular to each other while the transmitting axis ofthe lower polarizer 23 and the transmitting axis of the upper polarizer43 are parallel. Furthermore, Only the component of the light incidentfrom the back light unit parallel to the transmitting axis (X) of thelower polarizer 23 passes through the lower polarizer 23 and the firstliquid crystal layer 50 when the horizontal electric field is applied tothe first panel part (A). The light passes through the first liquidcrystal layer 50 which is 45° on average to the transmitting axis (X) ofthe lower polarizer 23 and its phase is delayed by λ/2. Therefore, thepolarization of the light (polarization to X axis direction) is changedto 90°. The light changed to 90° at the initial incident polarizationdirection passes through the transparent second substrate 31. The lightwhich passes the first liquid crystal layer 50 also passes through themiddle polarizer which is perpendicular to the transmitting axis of thelower polarizer 23. Then the light enters into the second liquid crystallayer 60 to which the vertical direction is applied. The light passesthrough the second liquid crystal layer 50 in the front direction (R)which is 45° to the transmitting axis (Y) of the middle polarizer andits phase is delayed by λ/2. Therefore, the polarization of the lighttransmitted in the front direction (R) is changed to 90° from thetransmitting axis (Y) of the middle polarizer. The light transmitted inthe front direction (R) has the same direction with the transmittingaxis (Y) of the upper polarizer 43 so that it passes through the upperpolarizer 43. Therefore, the liquid crystal display device displays abright image. However, when the light is transmitted in obliquedirections (S1, S2), the phase delay occurs due to the twisted conditionof the second liquid crystal layer 60 by the vertical electric field. Asa result, the polarized condition changes due to the phase delay.Therefore, the component of the light perpendicular to the transmittingaxis of the upper polarizer 43 (e.g., the light transmitted in obliquedirections S1 and S2) will be blocked by the upper polarizer 43.Therefore, the final display condition of the liquid display device isidentical to FIGS. 7A and 7B even if the second polarization substrateis added.

FIG. 8 shows a transmitting ratio of the liquid crystal display devicewith respect to the viewing angle when displaying a bright imageillustrated in FIGS. 6-7. As show in FIG. 8, the second panel part (B)which is ECB mode has a transmitting ratio illustrated as the γ-curvewhen the vertical electric field is off (wide viewing angle mode) andhas a transmitting ratio illustrated as the δ-curve when verticalelectric field is on (narrow viewing angle mode).

Referring to the γ-curve, when the vertical electric field is off (wideviewing angle mode) when displaying a bright image, the light passesnormally regardless of the viewing direction. The δ-curve illustratesthat the light is blocked as it is phase delayed at the obliquedirections against the second substrate due to the twisted second liquidcrystal layer when the vertical electric field is off (narrow viewingangle) when displaying a bright image.

FIG. 9 shows the contrast ratio curve at the wide and narrow viewingangle modes. Referring to FIG. 9, the W-curve shows the contrast ratioof the wide viewing angle mode through the α-curve of FIG. 5 showing thewide viewing angle transmitting ratio when displaying a dark image andthe γ-curve of FIG. 8 showing the wide viewing angle transmitting ratiowhen displaying a bright image. In addition, the N-curve shows thecontrast ratio of the narrow viewing angle mode through the β-curve ofFIG. 5 showing the narrow viewing angle transmitting ratio whendisplaying a dark image and the δ-curve of FIG. 8 showing the narrowviewing angle transmitting ratio when displaying a bright image isobtained. With reference to the W-curve, the contrast ratio at the wideviewing angle mode does not show a big decline at the right or leftoblique direction. Therefore, the wide viewing angle mode is obtained.On the other hand, referring to the N-curve, the contrast ratio at thenarrow viewing angle mode shows a big decline at the right and leftoblique directions and a clear contrast ratio can only be seen in thefront direction. Thus, the narrow viewing angle mode is obtained.

FIGS. 10A and 10B show the contrast ratio at the right/left andtop/bottom oblique directions of the wide and narrow viewing anglemodes. Referring to FIG. 10A, it shows that the range of viewing anglewhich has the contrast ratio of 10:1 is wide, although the brightness atthe oblique direction is somewhat lower than that of the frontdirection. FIG. 10B shows the range of viewing angle which has thecontrast ratio of 10:1 is narrowed.

The illustrated liquid crystal display device includes the first panelpart (A) of the IPS mode which decides the darkness/brightness of theimage and the second panel part (B) of the ECB mode which allowsswitching between the wide viewing angle mode and narrow viewing anglemode.

The dimension of the second panel part (B) of the ECB mode whichoperates the narrow viewing angle mode is as follows. When the cell gapof the second panel part (B) is 4 μm, the phase delay range is about14˜120 nm according to the tilt angle of the second liquid crystal layer60 at the application of the vertical electric field to the second panelpart (B). When the cell gap of the second liquid panel (B) is 3.4 μm,the phase delay range is about 14˜120 nm according to the tilt angle ofthe second liquid crystal layer 60 at the application of the verticalelectric field to the second panel part (B). The tilt angle range of thesecond liquid crystal layer 60 is between about 10° to 80° when thevertical electric field is applied to the second panel part (B). Thereason to set the tilt angle of the second liquid is between about 10°to 80° is because it is difficult to have an effective narrow viewingangle as the light leakage may be too much if the tilt angle of thesecond liquid crystal layer 60 is below 10°. It is also difficult tohave an effective narrow viewing angle at the range over 80° as thelight leakage may be too much. In order to have the tilt angle range ofthe second liquid crystal layer 60 to be between about 10° to 80°, theoperation voltage range (Δ v) of the second panel part (B) to generatethe vertical electric field should be about 1V<Δv≦4V.

The ECB mode panel is applied as the second panel part which controlsthe viewing angle of the liquid crystal display device. Instead ofapplying the ECB mode panel as the second panel part, the OCB (OpticalControlled Birefringence) mode panel may be applied. The OCB mode panelincludes at least one optical compensation film and has a highermanufacturing cost. It also requires large power consumption as acertain power needs to be applied to make the liquid crystal layer faceeach other in the middle of the liquid crystal layer at the initialstage of the liquid crystal alignment. In addition, the second panelpart controlling the viewing angle can be formed at the upper or thelower side of the first panel part.

As described above, the illustrated liquid crystal display device allowsswitching between the wide viewing angle mode and the narrow viewingangle mode as it comprises the first panel part of the IPS mode whichdecides the darkness/brightness, and the second panel part of the ECBmode at the upper or lower part of the first panel part to allowswitching between the wide and the narrow viewing angle modes.

The illustrated liquid crystal display device uses the IPS mode toprovide a wide viewing angle range and uses the ECB mode to change theviewing angle range of the liquid crystal display device. Therefore, themanufacturing steps of the LCD are simplified.

It should be understood that the invention is not limited to theembodiments. Various changes or modifications can be made under thecondition that those changes or modifications do not depart from thespirit of the invention. Accordingly, the scope of the invention shallbe determined only by the appended claims and its equivalents.

What is claimed is:
 1. A method for driving a liquid crystal displaydevice, the method comprising: displaying a video image on a first panelby selectively applying a horizontal electric field to a first liquidcrystal layer of the first panel of the liquid crystal display device;and selectively applying a vertical electric field to a second liquidcrystal layer of the second panel of the liquid crystal display device,wherein when the vertical electric field is applied, a tilt angle of thesecond liquid layer is from 10° to 80°, a driving voltage for generatingthe vertical electric field is from 1V to 4V, and a phase delay range ofthe light passing the second layer is from 14 nm to 120 nm.
 2. Themethod of claim 1, wherein the step of selectively applying thehorizontal electric field includes applying a voltage between a pixelelectrode and a common electrode of an in plane switching liquid crystaldisplay panel.
 3. The method of claim 1, wherein the step of selectivelyapplying the vertical electric field includes applying a voltage betweena first electrode and a second electrode of an electrically controlledbirefringence liquid crystal display panel.
 4. The method of claim 1,further comprising setting an initial alignment direction of the firstliquid crystal layer to be parallel to an initial alignment direction ofthe second liquid crystal layer.
 5. The method of claim 1, wherein thecell gap of the second panel is 3.4 μm or 4 μm.
 6. A method for drivinga liquid crystal display device, the method comprising: selectivelyapplying a first electric field in a first direction on a first liquidcrystal layer of the liquid crystal display device; and selectivelyapplying a second electric field in a second direction on a secondliquid crystal layer of the liquid crystal display device, the seconddirection being different from the first direction, wherein when thesecond electric field is applied, a tilt angle of the second liquidlayer is from 10° to 80°, a driving voltage for generating the secondelectric field is from 1V to 4V, and a phase delay range of the lightpassing the second layer is from 14 nm to 120 nm.
 7. The method of claim6, wherein the first electric field is selectively applied on the firstliquid crystal layer based on image data to be displayed on the liquidcrystal display device.
 8. The method of claim 6, wherein the secondelectric field is selectively applied on the second liquid crystal layerto change a viewing angle range of the liquid crystal display device. 9.The method of claim 6, wherein the second direction is substantiallyperpendicular to the first direction.
 10. The method of claim 6, whereinthe first direction is a horizontal direction and the second directionis a vertical direction.
 11. The method of claim 10, wherein the step ofapplying the first electric field includes applying a voltage between apixel electrode and a common electrode of an in plane switching liquidcrystal display panel.
 12. The method of claim 10, wherein the step ofapplying the second electric field includes applying a voltage between afirst electrode and a second electrode of an electrically controlledbirefringence liquid crystal display panel, wherein the first electrodeand the second electrode face each other in the vertical direction. 13.The method of claim 6, further comprising setting an initial alignmentdirection of the first liquid crystal layer to be parallel to an initialalignment direction of the second liquid crystal layer.
 14. The methodof claim 6, wherein the cell gap of the second panel is 3.4 μm or 4 μm.